You must Equate Something. In this case, you can equate Glucose or Water
G = Volume of 30% Glucose Solution
W = Volume of Water
G(0.3) + W(0.0) = 7(0.2)
G(0.7) + W(1.0) = 7(0.8)
With G + W = 7, each leads to the same solution.
Looking at your solution, you seem to have (Glucose Ratio) + Water = Glucose. That's no good.